How to get rid of this artifact when sculpting by using multires
Use Apply Base in Multires modifier's options to avoid possible artifacts for the base geometry of the mesh.

What it does is it tries to make base (unsubdivided) mesh to match sculpted mesh as much as its geometry allows. The artifact on the screenshot probably was caused by relatively big shift of the sculpted mesh regarding the base one (although generally it shouldn't happen but if it does try to make base and sculpted match as much as possible).